Natural gas, coal, and biomass produce syngas via a reforming reaction, and the produced syngas is used for synthetic raw materials of chemical compounds, fuels, and industrial processes by undergoing various downstream processing.
In addition, the produced syngas contains a large amount of hydrogen, and the hydrogen is used in the ammonia synthesis, refinery process, smelting process, polysilicon manufacturing process, semiconductor manufacturing process, LED manufacturing process, etc. after undergoing a purification process and is thus an essential substance in modern industry. Recently, research for using hydrogen in the smelting process even in the steel industry is currently in progress in order to achieve the goal of reducing carbon dioxide.
In particular, the value of hydrogen is constantly growing as a clean energy source having a high efficiency and no exhaust pollutants when used in conjunction with a fuel cell.
However, the preparation process of syngas in a series of processes, which produce hydrogen from natural gas, coal and biomass and use the same, accounts for 60% to 70% of the total cost of production, and thus, there is a need to develop a preparation process of syngas with excellent efficiency.
Meanwhile, the development of small- and medium-scale hydrogen production plants with a gas flow rate of 50 Nm3/h to 5000 Nm3/h for supplying hydrogen in the field is actively underway to diverge from the existing method of supplying by transportation, which is used in various industrial facilities such as those for ammonia synthesis, oil refinery processes, semiconductor manufacturing processes, LED manufacturing processes, polysilicon manufacturing processes, the iron and steel industry, etc.
The essence of the small- and medium-scale hydrogen production plants is that they must be economical compared to a transportation method based on mass production, and that they must enable flexible operation according to the circumstances of consumers.
The small- and medium-scale hydrogen production plants with a gas flow rate of 50 Nm3/h to 5000 Nm3/h are a technology which can compete with liquid hydrogen transportation and electrolysis, and the development of a process with excellent efficiency is absolutely necessary in order for the method of reforming natural gas to secure economic feasibility.
In addition, in order to take advantage of natural gas as a clean energy source although the natural gas is fossil fuel, the problems with carbon dioxide emission, which is the main cause of global warming, must first be resolved. Clean energy production technology using natural gas may be completed by collecting carbon dioxide before the release thereof to the atmosphere concurrently with the hydrogen production (pre-combustion CCS).
The method for preparing syngas from natural gas is largely classified into steam reforming of methane (SMR), partial oxidation of methane using oxygen (POX), carbon dioxide reforming of methane (CDR), steam carbon dioxide reforming of methane (SCR), in which steam reforming reaction and carbon dioxide reforming are combined, etc. The ratio of carbon monoxide and hydrogen (H2/CO) produced from each reforming reaction may be different, and thus the various reforming reactions may be used depending on the ratio optimally required in the subsequent process.
Meanwhile, the hydrogen production process by the conventional reforming reaction of natural gas is composed of SMR (700° C. to 900° C.)-HTS (300° C. to 450° C.)-LTS (200° C. to 250° C.)-PSA, as shown in FIG. 1a. The reformer used in the conventional method of the hydrogen production process requires a high operating temperature in the range of 700° C. to 900° C., and thus has the disadvantages of low operating efficiency and low economic feasibility owing to being composed of the high-temperature materials. Further, the conventional method of the hydrogen production process is carried out in separate reactors, and therefore, it is difficult to design a compact process.
Steam methane reforming (SMR) is a reaction which reforms natural gas in the presence of water vapor using a catalyst and then chemically converts the same into syngas (mixture gas of CO+H2) as shown in Reaction 1 below.CH4+H2O→CO+3H2 ΔH=206.28 kJ/mol  [Reaction 1]
SMR has a CO2/H2 ratio of 0.25 among the gas produced, and shows advantages in that it has a low CO2 production ratio and enables a larger amount of hydrogen to be obtained from a certain amount of hydrocarbons, compared to partial oxidation using hydrocarbons as raw materials.
The fluid produced from the SMR process contains a high CO/H2 ratio, and thus, CO may be converted into CO2 and H2 by a shift reaction as shown in Reaction 2 below. This is known as water-gas shift reaction (WGS).CO+H2O→CO2+H2 ΔH=−41.3 kJ/mol  [Reaction 2]
The shift reaction can be divided into a high-temperature shift reaction and a low-temperature shift reaction depending on the temperatures.
Therefore, the SMR process may lead to the high-temperature shift reaction (HTS) and the low-temperature shift reaction (LTS) thereafter.
The high-temperature shift reaction may be performed at 350° C. to 550° C. using Fe2O3 as a catalyst in which Cr2O3 is added as a co-catalyst. The chemical composition of a typical catalyst used is Fe (56.5% to 57.5%) and Cr (5.6% to 6.0%). In general, the high-temperature shift reaction has a high CO conversion rate as the space velocity decreases, and the reaction rate increases as the diameter of catalyst particle decreases. H2S does not affect the catalyst reaction in a significantly broad temperature range, but even a small amount of H2S decreases the rate of the high-temperature shift reaction if the temperature decreases. That is, H2S with 4% concentration does not affect the reaction rate at 410° C. or higher, but H2S with a concentration only within 0.5% does not affect the reaction rate at 350° C.
The low-temperature shift reaction is performed at 200° C. to 250° C., and uses catalysts such as CuO (15% to 31%)/ZnO (36% to 62%)/Al2O3 (0% to 40%). Cr-based low-temperature shift catalysts have recently been developed. The minimum reaction temperature must be higher than the dew point of water gas, and the concentration of CO in the exhaust gas becomes 1% or less. The low-temperature shift catalysts are used once they are converted to a reduced state through an initial activation process. Since H2S causes severe deactivation, it is necessary that a H2S removal process be carried out in the beginning of the low-temperature shift reaction such that the concentration of H2S is maintained at 0.1 ppm or less.
The shift reaction above may lead to a hydrogen purification process thereafter. The hydrogen purification process may include not only PSA (pressure swing adsorption), but also a membrane separation method, a cryogenic method, etc. For example, PSA may be composed of 4 to 12 adsorption beds. A molecular sieve may be used as an adsorbent, and 80% to 92% of H2 may be separated in the mixed gas flow of 400 psig to 500 psig. After the completion of adsorption, the adsorbent may be regenerated by reducing the pressure to 5 psig through a purge process using H2.
Under the circumstances above, the present inventors prepared a shell-and-tube type reactor for reforming natural gas equipped with at least one tube for separating hydrogen, and a tube for an exothermic reaction or a tube type heat-exchanger for heating, which is disposed at the center of the reactor, and in which a reaction catalyst for reforming the natural gas is filled in a reactor shell, and found that the reactor has excellent operating efficiency upon examination thereof and enables production of high-purity hydrogen and collection of carbon dioxide simultaneously with the reaction, thereby completing the invention.